Amide waxes have been used for many years as slip agents in the production of films. Chemically, the waxes are primary, secondary, tertiary, or bis fatty amides, such as oleamide and erucamide.
Amidic waxes are generally immiscible with the host polymer with which they are typically associated via an extrusion process. Host polymers typically include polypropylene, ethylene/alpha olefin copolymer, ethylene/vinyl acetate copolymer (EVA) or other ethylene copolymers such as ionomer resin, ethylene/acrylic or methacrylic acid copolymer, ethylene/acrylate or methacrylate copolymer; or low density polyethylene (LDPE).
Producers of polymeric film operate on the principle that the wax molecule is of a lower molecular weight than the host polymer, and therefore generally more mobile than the host polymer. When the host polymer is in a molten state (during extrusion), waxes can migrate more freely through the host polymer. Waxes migrate through a host polymer in solid state as well, but at a much slower rate. This wax migration, and the polar amide component of the wax molecule, leads to the phenomenon that the wax will diffuse to the surface of the polymeric film. The diffusion rate for surface migration is the “bloom rate”. The bloom rate is influenced not only by the relative molecular weights of the wax and host polymer, but also by the level of saturation of the wax hydrocarbon chain, the polarity of the host polymer, and the architecture of the secondary, tertiary, and bis fatty amide molecules.
Producers of polymeric film also operate on a second principle that packaging applications typically require a film with a low coefficient of friction (COF). This requirement is dictated by the need for the film to run properly on packaging equipment used by food processors and other packagers. For example, in the case of Vertical Form Fill Seal (VFFS) equipment, typical film requirements are a film-to-film COF of less than 0.3. Low molecular weight amidic waxes require sufficient time to “bloom” to the surface before a low COF is achieved in the final film. The resin choice for the surface layer of a VFFS film must have sufficient heat seal properties. Thus, sealant layers typically include those resins mentioned above, such as LLDPE, metallocene catalyzed polymers, and EVA polymers. These materials are much tackier than polypropylene (PP) or propylene/ethylene copolymer (EPC), and present a challenge for the development of adequate film surface properties such as COF or slip.
To help reduce this surface tackiness or tendency for the polymeric film to stick to itself or block, antiblock agents are typically utilized in the surface layer. Typical particulate antiblock agents include various silicas, carbonates, synthetic particulates, etc. Other types of antiblock agents include amidic waxes themselves such as the bis fatty amides. While such antiblock agents can reduce tackiness, they may not yield a high slip polymeric film.
For some packaging applications, the polymeric film is extruded in annular form, one or both edges of the film are slit, and the planar film is either wound or centerfolded in a manner well known in the art. A cylindrical forming piece is used to cast the molten polymer into an initial film thickness and diameter in the shape of a tube. The molten polymer is quenched to solid state by a water bath or spray, as it travels over the forming cylinder, into a continuous length of precursor film tubing.
A problem encountered in this process is that at the extrusion die, a significant amount of wax migrates to the surface of the just emerging tubing as the precursor film contacts film making equipment located downstream of the extrusion die. If wax has been added to the host polymer in the extrusion process at a level to provide the adequate (low enough) COF and adequate (high enough) slip properties needed in the final heat shrinkable film, the equipment becomes covered in wax over time. Such waxy deposits eventually break off in solid form or transfer to the tube in liquid form, resulting in aesthetic anomalies in the final film. Although aesthetic or cosmetic anomalies in the film are sufficient by themselves to affect the commercial viability of the film, at some level of contamination, the physical properties of the film can be compromised as well. Thus, frequent cleaning of the film making equipment may be required, resulting in undesirable downtime. Film end use governs the frequency of the cleaning interval, thereby limiting continuous extrusion to relative short periods of time, and increasing the cost of film production.
A second problem associated with the production of many polymeric films intended for packaging applications is that after the precursor film forming step described above, additional orientation steps are completed and a resulting heat shrinkable film is typically wound up in roll form. Amidic waxes such as erucamide often require a minimum of 24 hours to approach a sufficient equilibrium concentration on the film's outer surface (i.e. often require a minimum of 24 hours to bloom to the film's outer surface) to permit adequate processing on packaging equipment. When polymeric films are made, especially heat shrinkable films, the film is rolled up such that the bottom (that is, the interior winds near the film core) of a film roll can be under a relatively high tension of several hundred pounds per square inch. This roll pressure has been found to retard or even prevent significant wax from blooming to the film surface after the film is wound into roll form. Additionally, a film roll which is wound under high pressure may result in wax diffusion through several film layers thereby depleting wax concentration in some regions of the roll and buildup in other regions. Wax migration in the roll undermines desired consistent film performance from the beginning to the end of the roll. Consequently, the level of wax on the film surface has been shown to be greater on the top of the roll (i.e. the exterior winds of the roll) where roll pressure is less, than it is at the end of a roll nearest the metal, paper or plastic roll core member. This significantly affects the ultimate performance of the film, especially with respect to the manner in which the film runs or “tracks” on high-speed packaging equipment. Additionally, in some cases only portions of a roll of film may be utilized on packaging equipment, and the rest has undesirably “blocked”, i.e. bonded together at film surfaces on the roll, resulting in significant roll footage waste.
Applicants have found that a solution to the above-mentioned first problem (the build up of wax during the film making process) is to dispose one or more amidic waxes in an inner layer of a multi-layer film. This solution results in either no amidic wax present in the surface layers of the multi-layer film, or else significantly less wax present in the surface layers than in one or more inner substrate layers during the initial stages of film formation.
Applicants have found that a solution to the second above-mentioned problem (the differential level of surface wax within the rolled film) is to include a transition metal salt of stearic acid, or an ester of stearic acid, in the inner substrate layer or layers, and/or the surface layers of the film. Applicants have found that a transition metal salt of stearic acid, or an ester of stearic acid, acts as a facilitator to cause the wax to bloom more quickly to the surface of the film. Such accelerated bloom provides a multi-layer film with desired surface properties. Thus, wax in an inner substrate layer diffuses to the film's outer surfaces before the film is wound onto a roll, thereby providing adequate slip prior to the pressures encountered during winding of the film onto a core member. Such controlled bloom time can reduce the need for above-mentioned wax migration within a film roll and result in more uniform levels of wax at the film surface throughout a roll length.
The two solutions in suitable combination provide a polymeric multi-layer film, such as a heat shrinkable film, having good slip (i.e. low COF) properties, while eliminating significant wax build-up on film-making machine components. The ability to control wax migration reduces manufacturing waste and provides improved performance by ensuring consistency throughout rolls of polymeric films.